Embodiments of the invention relate generally to primary transformers such as a motor and, more particularly, to measuring current flowing through a primary transformer for use in detecting motor operational status.
In general, three-phase induction motors consume a large percentage of all generated electrical capacity. Many applications for this “workhorse” of industry are fan and pump industrial applications. Due to the prevalence of motors in industry, it is paramount that the three-phase motor be reliable. Industry reliability surveys suggest that motor failures typically fall into one of four major categories. Specifically, motor faults typically result from bearing failure, stator turn faults, rotor bar failure, or other faults/failures. Within these four categories: bearing, stator, and rotor failure account for a majority of all motor failures.
Detection devices have been designed that generate feedback regarding an operating motor. The feedback is then monitored to determine the operating conditions of the motor. A common industrial practice is to measure current using the same principles as a transformer. A magnetic field is induced around a conductor as current is passed through the conductor. This magnetic field may be induced into a magnetic coil looped around the conductor. This method is similar to an air core transformer and is commonly referred to as a current transformer. The amount of magnetically induced current into the coil is dependent on the number of coil loops and the amount of signal current desired. The current signal, therefore, should be proportional to the actual current in the conductor of interest. A scale is developed to read the coupled current signal value in the conductor as an actual current signal.
The output of the current transformer may be used to sense an overcurrent condition in the motor by sensing a high current passing through the conductor, for example. Once detected, the overcurrent condition may be set up to drive an overload relay to shut off the motor. Sensing a high current for overcurrent protection includes sensing a wide range of current that may pass through the conductor. The sensing circuit may have to provide feedback for a range of up to six times the normal operating range of the motor for overcurrent protection such as, for example, 750 amps.
Often, the sensed current is converted from an analog signal to a digital signal. Sensing and converting such a wide range of current to a digital signal results in a large step size between each digital value. For a circuit providing overcurrent protection, such a coarse digital step size scale is often adequate to provide motor protection prior to conditions leading to motor failure. However, besides sensing the motor current for purposes of overcurrent protection, it may be desirable to sense the currents flowing through the motor while the motor is operating in its steady-state mode of operation. In this case, the coarse digital step size of the scale set up for overcurrent protection is often too large to provide reliable accurate information when used for both overcurrent feedback as well as steady-state operation feedback.
Typically, current transformers that are used in sensing the motor current through the conductor are costly, heavy, and occupy a large amount of space within the system. Accordingly, simply adding an additional current transformer with associated sensing circuit into a system for each desired current sensing range is prohibitive.
It would therefore be desirable to have a system for sensing primary transformer current that provides multiple sensing ranges.